• Infants who have almost died of sudden infant death syndrome (SIDS) and infants who are siblings of SIDS victims constitute groups at increased risk for SIDS. Management dilemmas are common among physicians caring for these infants. To assess the usefulness of hypoxia (17% oxygen) and hypercapnea (3% carbon dioxide) challenge tests as predictors of outcome, we reviewed the records of 102 infants who underwent these tests. During hypoxia tests, we found that periodic breathing and respiratory pauses frequently developed among the infants in these high-risk groups, but also developed among control infants. During hypercapnea testing, some infants failed to increase their minute ventilation (usually measured by volume of breath × breaths per minute), but control infants showed this poor response just as often as high-risk infants. Our findings suggest that hypoxia and hypercapnea stress tests are of limited usefulness in planning management of infants at risk for SIDS.